Review Article Physical and Chemical Modifications of Plant Fibres for Reinforcement in Cementitious Composites
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Hindawi
Advances in Civil Engineering
Volume 2019, Article ID 5185806, 18 pages
https://doi.org/10.1155/2019/5185806
Review Article
Physical and Chemical Modifications of Plant Fibres for
Reinforcement in Cementitious Composites
R. Ahmad ,1 R. Hamid ,1,2 and S. A. Osman1,2
1
Smart and Sustainable Township Research Centre, Faculty of Engineering and Built Environment,
University Kebangsaan Malaysia, 43600 UKM, Bangi, Selangor, Malaysia
2
Civil Engineering Programme, Faculty of Engineering and Built Environment, University Kebangsaan Malaysia,
43600 UKM Bangi, Selangor, Malaysia
Correspondence should be addressed to R. Hamid; roszilah@ukm.edu.my
Received 14 November 2018; Revised 31 January 2019; Accepted 12 February 2019; Published 12 March 2019
Academic Editor: Giosuè Boscato
Copyright © 2019 R. Ahmad et al. This is an open access article distributed under the Creative Commons Attribution License,
which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
This paper highlights the physical and chemical surface modifications of plant fibre (PF) for attaining suitable properties as
reinforcements in cementitious composites. Untreated PF faces insufficient adhesion between the fibres and matrix due to high
levels of moisture absorption and poor wettability. These conditions accelerate degradation of the fibre in the composite. It is also
essential to reduce the risk of hydrophilic PF conditions with surface modification, to enhance the mechanical properties of the
fibres. Fibres that undergo chemical and physical modifications had been proven to exhibit improved fibre-matrix interfacial
adhesion in the composite and contribute to better composite mechanical properties. This paper also gives some recommen-
dations for future research on chemical and physical modifications of PF.
1. Introduction PF also have a great potential to replace glass in many
applications [5], and they cause less dermal and respiratory
The incorporation of fibre in cementitious material com- irritation than glass fibre [6]. Other observations by several
posites as a reinforcement can enhance the flexural limit researchers also indicate that PF can yield the same flexural
during splitting, durability, ductility, and break resistance strength and a higher Young’s modulus compared to glass
compared to the unreinforced matrix [1]. In addition, the fibre [7–10]. The use of PF can easily be adopted in cement
benefit of fibre as reinforcement in cement composites is the composites for economic and environmental reasons if the
ability to control crack growth and to increase ductility [2]. hydrophilic nature, low processing temperature, wettability,
Much work has been carried out to identify the potential use incompatibility, and high moisture absorption of the fibres
of natural fibre (NF) compared to artificial fibre in are defeated. Hence, this paper reviews the challenges in
strengthening composites. Efforts have been made to utilize resolving the above issues of using PF in composite. The
NF-reinforced composites as part of building material in the research on the physical and chemical treatments of PF to
construction industry. By exploiting the benefits of NF due to enhance its mechanical and physical properties is ongoing so
its lower density, tool wear, and cost, NF has overtaken ar- that PF can compete with other synthetic fibre-reinforced
tificial fibre in many applications and is well suited for use as a composites and wood products and bring many potential
reinforcement either in a cement matrix or in polymer applications in the construction industry [11].
composites [3]. Additionally, the renewable and biodegrad-
able characteristics of NF make the fibres easy to dispose of by 2. Natural Fibres
composting or incineration compared to artificial fibre.
More recent evidence shows that sustainability, re- Fibres are classified according to their origin and are divided
newable sources, and broad use of NF have introduced into synthetic fibre and NF. NF is extracted from plant, animal,
several plant fibres (PF) into the biocomposites field [4]. and mineral sources [12]. Examples of fibre classification are
2 Advances in Civil Engineering
Fibres
Natural Man-made
Plant Mineral Synthetic Artificial
(i) Asbestos (i) Viscose
(ii) Rayon
Organic (iii) Modal
Nonorganic
Animal (iv) Acetate
(i) Polyethylene
(i) Glass, carbon (v) Triacetate
(i) Silk (ii) Polyester
(ii) Boron
(ii) Wool (iii) Polypropylene
(iii) Silicon
(iii) Hair (iv) Acrylic
(iv) Carbide
(v) Modacrylic
Bast
Leaf Stalk
(i) Abaca Grass
(i) Sisal (i) Paddy Fruit
(ii) Flax Seed Wood
(ii) Curaua (ii) Wheat (i) Bamboo
(iii) Jute (i) Coir
(iii) Banana (iii) Corn (ii) Switch (i) Kapok (i) Softwood
(iv) Hemp (ii) Oil
(iv) Henequen (iv) Barley (iii) Grass (ii) Cotton (ii) Hardwood
(v) Kenaf (iii) Palm
(v) Agave (v) Oat (iv) Miscanthus
(vi) Ramie
(vi) PALF (vi) Rye
(vii) Rattan
Figure 1: Fibre classification.
S3 rather than combined with individual fibres for composite
applications [16]. The chemical composition of PF after surface
S2 modification has a strong impact on the mechanical properties
Secondary wall
Cellulose crystalline by removing lignin and hemicellulose [17], which are re-
microfibrils
S1 sponsible for the bonding behaviour and degradation of PF in
composites.
In addition, the properties of PF also vary depending on
Cellulose amorphous Primary wall the internal structure, fibre diameter, microfibril angles, cell
microfibrils (lignin dimensions, crystal structure, and defects [18]. Other factors
and hemicellulose) influencing the PF properties are different fibres taken from
different parts of plants, such as fibres extracted from the
stem, leaf, or seed, and different growing conditions [19]. The
Figure 2: Schematic representation of PF structure (source: [13]). advantages and disadvantages of bast fibre are shown in
Figure 3. Based on the weaknesses of PF shown in Figure 3,
shown in Figure 1. The fibres extracted from plants are further researchers have initiated detailed studies to identify the effect
classified into bast, leaf, stalk, fruit, grass, seed, and wood. of physical treatment on PF for improvement in its me-
Figure 2 shows the PF structure of the amorphous cellulose chanical properties and to improve the properties of the
held together by a lignin and hemicellulose oriented randomly composite materials. Table 1 shows that physical treatments
in cell walls. Hemicellulose, lignin, pectin, and the waxy contribute significantly to enhancing the fibre tensile strength
substances contained in lignocellulose are the utmost chemical and the mechanical properties of the polymer composites.
components in PF [14]. The hemicellulose provides cementing However, fibre strength is not the dominant factor related to
material in the cell wall and forms a matrix surrounding the composite strength, whereas good fibre orientation/
cellulose microfibrils, whereas the amorphous lignin gives dispersion and excellent bonding between the fibre-matrix
additional strength and coupling to the hemicellulose-cellulose proved to be the promoting factors in accelerating the me-
network, which becomes a protective barrier in fibres [15]. chanical properties in flexural and tensile strength [27]. The
From Figure 2, the crystalline cellulose microfibrils in sec- most important limitation lies in the hydrophilic nature of
ondary walls (S2) determine the fibre mechanical properties cellulose fibre that affects the mechanical properties and
and provide good mechanical properties when used alone performance of PF in composites [28].
Advances in Civil Engineering 3
Disadvantages Advantages
High
High
moisture
strength
absorption
Strong Renewable
hydrophilic Bast carbon-neutral
character fibres natural resources
Incompatible
with some Lower
hydrophobic Alternatives environmental
matrices to glass fibres impacts during
production
Recovery Improve fuel
energy and efficiency and
carbon afterlife reduce emissions
Figure 3: Advantages and disadvantages of bast fibres (source: [20, 21]).
Table 1: Effect of PF physical treatment on the mechanical properties of composite materials.
Mechanical properties
Fibres
Matrix (MPa) Ref.
Types Treatment Tensile (MPa) Compressive Flexural
Jute Argon cold plasma — Unsaturated polyester resins NA 180 [22]
Arundo Donax Plasma 192.32 Epoxy composites NA 75 [23]
Jute Ionized air 547 Phenolic composites 37.5 NA [24]
Piassava Electron beam — Composites 23 17 [25]
Kraft Fibre beating 45 PP composites 41 NA [26]
PP, polypropylene, NA, not available.
3. Composites used in composites, the interest in NF as reinforcement has
grown because of its low cost, weight reduction, nontoxicity,
The cement mortar composite material should be developed ease of recyclability, and biodegradability [34, 35].
to acquire properties that cannot be achieved by any of the
materials independently. Shanks [29] has indicated that the 4. Natural Fibre-Reinforced Composites
reinforcement and matrix are the two main constituents in
composites, where reinforcements are fibre structures that Hassanin et al. [36] stated that the largest advantages of
give strength to the materials and the latter surrounds the using NF in cement composites are the low cost of materials,
fibres with elastic interaction that holds them in place, sustainability, and density. The properties of fibre-reinforced
transfers force between the fibres, and resists a small portion composite materials are controlled by a number of factors as
of the loading [30]. The combination of the properties of follows: (1) magnitude and proportion of the fibre-matrix
cement and fibrous materials treated with chemical can elasticity; (2) type and properties of the matrix, such as
contribute in the performance of mortar as building material ductile or brittle; (3) fibre content, length, and orientation;
[31]. Mishra et al. [32] found that the performance of the and (4) interfacial bond strength in the fibre-matrix [37].
composites can be enhanced persistently through thorough However, the bond strength solely depends on the effec-
experimentation by blending two or more fibres or fillers, tiveness of the fibre as reinforcement in composites [38].
and Kwon et al. [33] indicated that the composite strength Due to poor interfacial bonding between the hydrophilic
and stiffness are transferred by the fibres. Although synthetic fibres and the hydrophobic polymer matrices, Romanzini
fibres are the most common type of fibre reinforcements et al. [39] conducted a study and found that the addition of
4 Advances in Civil Engineering
treated stiff fibres in composites can produce a new material PF has potential for use as a reinforcement material in
with outstanding mechanical properties. Other observations cement composites, but it is compulsory to utilize the strong
by Bentur and Mindess [40] confirm that adding fibres can reinforcing of PF in order to produce high mechanical
enhance the abrasion resistance, impact resistance, and fa- strength for composite materials [54].
tigue characteristics of cementitious composites.
However, fibre-reinforced cement composites tend to be 6. Fibre Surface Properties
exposed to degradation during wet/dry cycles from
accelerated ageing. This condition will affect the durability of Surface properties are generally defined as the surface free
cementitious composites. However, according to Mohr et al. energy that is used for characterizing the interaction be-
[41], the degradation of fibre-reinforced cement composite tween solid surfaces related to the adhesion properties of
can be mitigated with fibre and matrix modifications. Fibre materials [55]. Findings of Dai and Fan [56] show that the
modifications will stimulate chemical composition, di- surface properties of a fibre are the main factors that affect
mensional stability, and bond strength in PF cementitious the interfacial adhesion on the surface of fibres and the
composites. Meanwhile, matrix modifications containing mechanical properties of the composite reinforced with PF.
blended cementitious materials as a partial replacement of Adhesion is needed since the fibres and matrices are
OPC are effective in preventing degradation [42]. This chemically dissimilar, and Valadez-Gonzalez et al. [57]
suggests that the future of PF-reinforced composites seems highlighted that the fibre surface must consequently be
to be bright because they are cheaper, lighter, and envi- modified to improve the properties and interfacial in-
ronmentally superior to glass fibre composites [43]. Since teraction of PF for use as a reinforcing material in
strength and water absorption are the two main factors that composites.
limit the performance and the urge to increase the future use However, bast fibres will degenerate easily in the alkaline
of PF-reinforced composites, a thorough discussion cover- environment of the cementitious matrix, giving low impact
ing physical and chemical treatments for modification of PF strength and brittle composites [58]. Cordeiro et al. [55]
will be presented. investigated the surface properties of raw and modified lig-
nocellulose fibres by inverse gas chromatography (IGC)
5. Mechanical Properties of Plant Fibres treatments and found that bast fibres offered higher surface
dispersive energy compared to leaf fibre. Later, Praveen et al.
Strengthening mechanisms have triggered interest among [59] reported that plasma-induced modification changed the
researchers to enhance the mechanical properties such as surface topography and generated high water absorption of
compression, tensile, flexural, or impact strength, and wear coir fibres. The test indicates that different surface property
behaviour that signify the high achievement of good ma- treatments resulted in different interfacial shear strength.
terials. There is a strong indication that the fibre-matrix Thus, improvement in fibre surface properties is needed and is
properties are important in improving mechanical prop- dependent on the following factors: (1) fibre morphology, (2)
erties that determine the capability of material under ex- chemical composition, (3) extractive chemicals and pro-
treme loading as well as in critical conditions, and this cessing conditions, and (4) modification of plant fibres [60].
pointed to the performance of the composites [7]. Studies
have been performed by Fazal and Fancey [44] on PF- 7. Fibre Surface Modification Methods
reinforced composites, and they have discovered that the
mechanical properties of the composites are strongly Fibre surface modification can improve fibre-matrix in-
influenced by a number of parameters such as the volume terfacial bonding, roughness, wettability, and the hydro-
fraction of the fibres, the fibre length, orientation and aspect philic nature and can decrease moisture absorption, which
ratio, adhesion in the fibre-matrix, and stress transfer at the can enhance the tensile properties of PF in cementitious
interface. composites [46]. However, the impurities and waxy sub-
When selecting a suitable fibre, the mechanical prop- stances that lay on the PF surface will develop poor surface
erties are extremely important and become a major element wetting and reduce bonding in the fibre-matrix [61]. Thus,
to resolve in using PF as a reinforcement in composites [45]. the PF will need to undergo surface treatment before its
Various studies have been carried out to determine the application as reinforcement in cement composites. On the
effects of physical and mechanical properties on PF, and the other hand, Fiore et al. [6] stated that physical or chemical
results displayed in Table 2 show that the tensile strengths of modification or a combination of both must be proceeded
bast and leaf fibres were the highest. As mentioned by to strengthen the poor properties of the PF surface by
Bledzki et al. [51], bast and leaf fibres are classified as hard reducing the polar component through (1) removal of
fibres and are suitable for use as reinforcements in com- impurities, (2) changing the crystallinity and chemical
posites as they give a high stiffness and strength to the composition, (3) improving the fibre-matrix interface, and
materials compared to other fibres. Hence, the technical (4) attaining good adhesion in the fibre-matrix. The fibre
brief of the fibre is also an important factor to determine the modification methods shown in Figure 4 can be divided
PF structure as well as the characteristics [52]. There are into three groups: (a) physical treatments to improve the
several physical properties that are significant in selecting a properties of PF, such as strength, modulus, and elonga-
suitable PF for use in composites, such as fibre structure and tion; (b) chemical treatments to improve the interfacial
dimensions, defects, crystallinity, variability, and cost [53]. properties of the fibre-matrix and the durability of the fibre
Advances in Civil Engineering 5
Table 2: Physical and mechanical properties of PF.
Group Fibres Density (g/cm3) Tensile strength (MPa) Tensile modulus (GPa) Elongation at break (%) Ref.
Abaca 1.5 400 12 3–10 [3]
Flax 1.5 345–1035 27.6 2.7–3.2 [3]
Jute 1.3 393–773 26.5 1.5–1.8 [3]
Bast Hemp 1.48 690 70 1.6 [3]
Ramie 1.5 560 24.5 2.5 [3]
Kenaf 1.45 930 53 1.6 [46]
Roselle 0.75–0.8 170–350 17 5–8 [47]
Sisal 1.5 511–635 9.4–22 1.5 [3]
Curaua 1.4 500–1150 11.8 1.4 [48]
Leaf Pineapple 0.8–1.6 400–627 1.44 0.8–1.6 [3]
Poplar 0.25–0.39 — 2.9–8.4 — [49]
Banana 0.75–0.95 180–430 23 3.36 [47]
Coir 1.2 593 4–6 4.4 [50]
Fruits
Oil palm 0.7–1.55 248 3.2 0.7–1.55 [3]
Cotton 1.51 400 12 3–10 [5]
Seed
Kapok 1.47 45–64 1.73–2.55 2–4 [18]
Sea grass — 453–692 3.1–3.7 13–26.6 [18]
Grass Bamboo 0.6–1.1 140–230 11–17 — [3]
Bagasse 1.25 222–290 27.1 1.1 [3]
Fibre modification
Physical treatment Chemical treatment
Physicochemical
treatment
Simple mechanical Alkali
(i) Stretching
(ii) Calendering
(iii) Rolling
Coupling agent
(i) Silane
(ii) Acylation
Solvent (iii) Graft copolymerization
extraction
Electric discharge Bleaching
(i) Corona (i) Reductive
(ii) Plasma (ii) Oxidative
(iii) Ionized
(iv) Thermal
(v) Ultraviolet (UV) Enzyme
(vi) Electron radiation
(vii) Fibre beating
(viii) Dielectric barrier
Peroxide
(i) Permanganate
Figure 4: Fibre modification method.
6 Advances in Civil Engineering
in cement-based composites; and (c) physicochemical increase the surface area and remove soluble impurities for
treatments that provide clean and fine PF or fibrils that short PF and fillers [95]. Solvent extraction method proved
have very high cellulose content [62–64]. that lignocellulosic fibres can be separated from PF sources
by selective solvent action, obtaining fibres with high con-
8. Physical Methods tent of cellulose. This treatment is an effective method to
separate a compound based on the solubility of blended
Physical methods used to treat PF can effectively change water with an organic solvent [96]. However, this treatment
structural and surface characteristics, improving thermal was not widely used because PF will encounter degradation
properties and influencing the mechanical bonding of the due to a decreasing fibre aspect ratio. In addition, during the
composites without changing the chemical composition of solvent extraction process, hazardous steam formed and
the PF [55, 56]. These modification methods are imple- contaminated the water by leaching into ecosystems, which
mented on PF for (1) separation of the fibre bundles into is harmful to the environment [64]. According to Płotka-
individual filaments and (2) improvement of fibre surfaces Wasylka et al. [72], many new bioderived solvents have been
for composite applications [65], and they can be divided into discovered lately but due to some specific requirements for
three main treatments: (1) mechanical treatment, (2) solvent solvents to be used for this application, apparently, not all of
extraction treatment, and (3) electric discharge treatment as it can be used.
shown in Figure 4. The advantages and disadvantages of fibre
physical treatments are summarized in Table 3. From Ta-
ble 3, it can be concluded that each physical treatment of- 8.3. Electric Discharge. The purpose of electric discharge of
fered different benefits in terms of increment in mechanical this treatment is to separate cellulose, increase the melt vis-
properties and surface area, high crystallinity, and improved cosity, and improve the mechanical properties of PF [19].
durability of the treated PF. Electric discharge is an appropriate treatment method to
improve the compatibility between hydrophilic fibre and
matrix through roughening of the fibre surface and structure
8.1. Simple Mechanical. Field of the application of different [19]. In contrast with solvent extraction, the electric discharge
types of physical methods used to treat PF is constantly method has a low impact on the environment [64]. Thermal
increasing and can be divided into simple mechanical, treatment is the most popular electrical discharge method to
solvent extraction, and electrical discharge methods. Simple change the physical properties of the material and preserve
mechanical treatment such as stretching, calendering, roll- the chemical composition of the PF [97]. Heating PF to
ing, or formation are conventional mechanical methods for temperatures between 100 and 200°C for various durations
surface treatment of long PF that influence the bonding of will separate the lignocellulose fibre bundles into single fil-
fibres with the polymer matrix. Stretching process has the aments due to dry-up [98]. Other noncellulose/chemical
potential to give a maximum tensile strength but the process constituents with lower glass transition temperatures or
also can trigger elongation in which the PF could glide over similar with lignin will release or depolymerize from the fibre
one another during stretching resulting in elongation and bundles [99]. Rong et al. [100] also recorded that the crys-
extra extension [89]. Calendering process with the applied tallinity of PF is increased when exposed to lower temperature
pressure by calender rollers converts the PF into uniform ranges due to fibre stiffness and improved physical adhesion
continuous sheet that can be trimmed and fitted into desired between fibre-matrix. The exact temperature for PF to in-
mould. Gupta and Gupta [90] stated that the calendering crease in crystallinity has been found at 150°C and concluded
process has fulfilled the objectives of improvement in the that thermal treatment resulted in higher strength and
surface smoothness and density of PF. Thus, the pore size has modulus compared to chemical treatments [101].
been reduced and larger particles are held back into PF sheet. Another way of fibre treatment with electrical discharge
Rolling and swaging are used to yield PF bundle separation methods is plasma treatment and it is very effective in
and the rolling effect resulting in enhancement of dispers- substrate surface activation for PF. The plasma treatment is a
ability and adhesion with polymeric matrices [91]. chosen method to limit the use of chemicals for surface
The conventional treatments can reduce the potential treatments due to increasing concern for environmental
loss to the fibres and increase the surface area for fibre- pollution [102]. This treatment is functional for optimizing
matrix interaction in the composite [92]. This contrasts with the fibre-matrix interface of polymer composites, and it was
Varshney and Naithani [19], who found that fibres tend to an effective and stable treatment to modify the surface of PF
tangle and lead to high energy consumption during treat- [103] without using a chemical solvent. This conventional
ment. However, the PF chemical structure will not change method only modifies the outer surface layers resulting in
through the entire process and will improve the performance significant changes of PF surface morphology [104] with
of the composite as reinforcement [93]. This finding was improved wettability.
later proved by Rana et al. [94] that the fracture energy of
composites using PF was considerably enhanced though it However, it was found that plasma-treated PF generated
did not give significant influence on strength and stiffness. lower strength value due to the degradation of the PF after
the treatment and is not advisable to be used as re-
inforcement in composites [22].
8.2. Solvent Extraction. Solvent extraction or partitioning is Corona treatment is a type of atmospheric plasma
the easiest method using mechanical fractionation that can technique along with dielectric barrier. Corona surface
Advances in Civil Engineering 7
Table 3: Advantages and disadvantages of fibre physical treatments.
Treatment Advantage Disadvantage Remarks Ref.
Better load distribution in High heating rate during stretching Heat treatment will improve the
Stretching composite as a result of a reduction can generate shrinkage on the fibre fibre strength and develop high [66, 67]
of fibre rigidity and density surface tensile modulus
Enhancement of surface area Fibres became tightly packed, and
available for matrix interaction by Fibres exposed to potential damage the filtration efficiency is increased
Calendering [68, 69]
yielding bundle separation of long during the process by regulating density and
fibres permeability
Fibres became plasticized and will
Longer processing will decrease Surface and structural properties of
enhance the performance of the
Rolling fibre performance and disallow the lignocelluloses fibre were [70, 71]
composite when added into the
matrix interface benefits partially changed
matrix
New renewable and biodegradable
Discharge from the solvent
High fibre cellulose content was bioderived solvents are of great
Solvent extraction process is harmful to the
extracted from plants due to interest and can contribute to an [72, 73]
extraction ecosystems and produces
removal of impurities environmentally sustainable solvent
environmental pollution
extraction
Fibre wettability improved through Usable as standalone surface
Corona fibre surface polarity and Surface ablation and etching can treatment or early treatment to
[74, 75]
treatment compatibility between hydrophilic reduce fibre firmness activate cellulose for chemical
fibres and the hydrophobic matrix modification such as grafting
Simple process without any
Degradation and changes on the
pollution to modify the surface of PF have stronger interaction with
fibre surface occur from an etching
Plasma NF without altering the bulk the matrix and enhanced the
mechanism that generates pits on [76, 77]
treatment properties of the fibres and increase mechanical interlocking of the
the fibre surface area exploiting the
in the wettability of the fibre-matrix fibre-matrix in composites
plasma properties
interface
Altering the NF structure by Mechanical properties and the Increases the wettability of fibre, but
Ionized air separating the fibre bundles performance of PF as a the interfacial property
[24, 78]
treatments through removal of impurities from reinforcement were affected by enhancements are lower compared
fibre surfaces longer ionized air treatment to chemical treatments
Longer thermal exposure decreases The combination of chemical and
Improved thermal stability,
Thermal the moisture content and changes thermal treatment can increase the
crystallinity, and physical and [79, 80]
treatment the physical and chemical initial strength and improve the
mechanical properties of the NF
composition of the fibre durability of fibre
Does not have much influence on The temperature is varied between
Steam Environmentally friendly and low
fibre strength, crystallinity, or 120–220°C with a variation in time [81, 82]
explosion cost
thermal stability period from 30 min to 2 h
Polarity of the fibre surfaces, Surface oxidation improved the
wettability, and fibre-matrix mechanical properties and adhesion
Ultraviolet
interfacial along with mechanical Fibre strength decreased between the fibre-matrix compared [83, 84]
(UV)
properties of biocomposites are to traditional electrochemical
increased treatments
Degradation of high cellulose
Improves the interfacial bonding of Development of composite
content under the effect of
Electron NF and matrix due to free radicals, materials with good mechanical
irradiation will reduce the [25, 85]
radiation which ensure crosslinking between properties and morphology but lack
maximum thermal decomposition
the fibre-matrix of durability
temperature
An effective treatment to mitigate
Improvement in terms of durability
the fibre degradation in composites Decrease the average fibre length,
Fibre beating but lower mechanical strength due [26, 86]
related to accelerating ageing curling, and tensile strength
to defibrillation
conditions
Surface roughness increases and
Fibre strength decreases due to
Increased the incorporation of leads to improvement in wettability
Dielectric primary decomposition and short
fibres and polymerization into the due to changes in fibre surface [87, 88]
barrier decolourization of low energy
matrix microstructure but a reduction in
consumption and time
tensile strength
8 Advances in Civil Engineering
treatment uses low-temperature corona discharge plasma to polymer, (2) additives, (3) temperature, (4) pressure, (5)
transmit changes in fibres properties and alter the surface dose, (6) dose rate, (7) morphology, (8) crystallinity, and (9)
characteristic of PF [74]. This method is an electrical dis- surface volume ratio [118]. Free radicals on the surface of the
charge applied on a surface energy of PF at or near atmo- PF generated from electron radiation process can initiate
spheric pressure using electric current that changes the grafting polymerization of functional groups and the
surface energy of the cellulose fibres [105]. Corona treatment modified PF can be used in composites to enhance the
are not widely used due to the difficulties to use on three- bonding between fibre-matrix, or used as an independent
dimensional fibrous materials [106], inherent complexity, functional material such as heavy metal ion adsorption and
and insufficient number of investigations dedicated to un- wastewater purification [119]. Hence, the irradiation will
derstanding their behaviour on PF [65]. However, these significantly transform the structure, reactivity, mechanical
physical methods offers many advantages such as no need properties, and physicochemical properties of cellulose
for specific conditions during modification, low-cost process [120].
with low energy consumption, and high volume of material It was found that fibre beating is a well-established process
can be applied in large scale during the treatment and can of physical surface modification in the paper industry that
benefit the industrial production line of PF [107]. induces better performance of the PF cementitious composite
Whilst, the dielectric barrier through the plasma [121]. Beating or refining is a mechanical treatment to develop
process provides a nonthermal, nonequilibrium plasma a controlled amount of smaller fibrils, improve fibre bonding,
and modifies surface properties of fibres at atmospheric and develop optimum strength in preparing the pulp for the
pressure [108] and similar with corona treatment process. paper manufacturing process. It is widely used in wood re-
The free electrons in the plasma discharge are heated up lated material but was implemented in PF as well [49]. In
to 10,000–100,000 K, while the gas itself can be kept at general, the three main effects of beating/refining are as
moderate temperatures between room temperature and follows: (1) internal fibrillation increases the flexibility of fi-
100°C [109]. High-energy electrons are generated through bres by the breakdown of fibre walls into separate lamellae, (2)
collisions during discharge and able to produce radicals external fibrillation is described as the creation and/or ex-
and electronically excited particles efficiently. The posure of fibrils on the surface of the fibres, and (3) the
treatment process can promote the surface activation of generation of fines from fibres when they are no longer able to
the PF, but the discharge is not completely uniform and sustain compressive and/or shear forces during the treatment
has a short duration [110]. Although the ionized air [122]. According to Li et al. [123], an optimum degree of
treatment using electric discharge is a similar method to beating at 60°SR (degree of beating) leads to higher porosity
corona discharge, the treatment will minimize the ag- and a greater extent of fibrillation. The PF fibrillation degree
gregation of fibre bundles [111] In addition, the sepa- obtained from the beating process enables the control of the
ration of the fibre bundles and penetration of the ionized microstructure and mechanical properties of PF [124].
air through the PF increases the PF loading and matrix The least popular physical method is ultraviolet (UV).
interactions at the interface during the treatment [112] UV rays are suitable for textile surface treatment with
and revealed significant changes of the surface roughness shorter wavelengths than visible light that ranges from 10 to
after the treatment, which could also enhance the PF 400 nm and can trigger chemical reactions with several
wettability. organic molecules [107], thus, leading to greater effects
The steam explosion treatment is one of the most effi- rather than simple heating effects. It was found that UV
cient methods for removal of hemicelluloses fibres with treatment resulted in a greater degree of surface oxidation
rapid process in addition to alkaline extraction [113]. The when compared to conventional electrochemical treatment
steam explosion methods require less energy consumption, [125]. Bast fibres such as raw hemp, flax/jute, kenaf, abaca,
hazardous chemical, human toxicity, and environmental and grey cotton provide good UV protection through
impact in comparison with the alkaline treatment in order to natural pigments, lignin, waxes, and pectin that act as UV
gain higher fibre yielding [114]. This method involves in radiation absorbers [126]. In addition, techniques involving
treating PF with saturated steam at various temperatures and UV has proved to be clean methods and are widely attractive
reaction times, activated by shear force generated from because UV sources are relatively cheap, flexible, and easy to
moisture expansion and acetic acid form the hydrolysis of install [127].
acetyl groups in hemicellulose obtained from PF [115]. This
type of treatment physically shatters the fibres from within to 9. Chemical Methods
release impurities and form fibrils without changes in
chemical composition. Degradation may occur in some The purpose of chemical treatment is to modify and ac-
cellulose fibres depending on the gradation of treatment, tivate the fibre structure using a hydroxyl group that can
time, and temperature [116], and the PF are not significantly change the composition of the material by introducing
damaged due to greater strength, crystal structure, and new elements to interact with the matrix [57]. Using
limited water uptake compared with the rest of the PF. chemical regents for fibre modification will increase the
The electron radiation method is a surface modification mechanical properties of the fibre and the strength of the
treatment applied to PF to develop interactions between PF fibre-reinforced cement composite and will improve the
and the polymeric matrix for property improvement [117]. adhesion between the fibre surface and polymer matrix by
The established reaction depends on (1) type of fibre/ reducing the water absorption of the composites. Alkali,
Advances in Civil Engineering 9
coupling agents, bleaching, enzymes, and peroxide are agents causes impressive improvements in the character-
among the chemical treatments that are reviewed here, and istic values of composites depending on the fibre, matrix,
the advantages and disadvantages of using these treat- and type of surface treatment used.
ments during fibre modifications on PF are presented in
Table 4.
9.2.1. Silanes. Silanes are efficient coupling agents and are
extensively used to promote adhesion to hydrophilic com-
9.1. Alkaline. Alkali treatment or mercerization is effectual, posites and adhesive formulations, and various types were
low cost, and the most commonly used chemical treatment found to have effectively modified the interface proper-
for PF modification. The treatment is a process to increase ties of the natural fibre-polymer matrix interface, wood-
the surface roughness by shattering the internal hydrogen polypropylene, mineral-filled elastomers, fibre-reinforced
bonding that changes surface topography, crystallinity, unit epoxies, and phenolic [13]. The chemical structure for si-
cell structure, moisture absorption, and orientation of fibrils, lane coupling agents generally consists of R(4-n)-Si-(R1X)n,
enhancing the mechanical properties of the fibre [146]. where R is alkoxy, R1 is an alkyl bridge connecting a silicon
During the treatment, lignin, wax, and oils that conceal the atom, and X is organofunctionality [158]. Nonswelling be-
exterior surface of the fibre cell wall will be partly removed, haviour, high chemical resistance, and an increase in tensile
as well as hemicellulose, and that will trigger the cellulose strength are the outcome of cross links between silanes, treated
decomposition and expose short length crystallites [147]. fibres, and matrix that accelerates the efficacy of composites
Table 5 displays the effect of alkali on the tensile properties of [159]. Trialkoxysilanes and c-aminopropyltriethoxysilane (APS)
NF. From Table 5, it is obvious that the tensile strength are the types of silanes that are frequently used as coupling
increase correlated with the percentage of alkali used in the agents to reduce the number of hydroxyl groups, forming
treatment, indicating that severe or more than 10% alkali silanols that are adsorbed on to the fibre-matrix surface [160].
resulted in weakening or damaging the fibre and reduction The fibre-matrix interaction depends upon the organo-
in fibre tensile strength. A mild alkali added between 6 and functionality of silanes and the matrix.
9% will increase the fibres tensile strength approximately
30% compared to untreated fibres.
Several researchers conducted studies to analyse the 9.2.2. Acylation. Acylation or esterification methods are
correlation between sodium hydroxide (NaOH) solution and divided into acetylation (using acetate) and valerylation
the mechanical properties of both PF and PF-reinforced (using valerate) for plasticizing PF. An acetyl group during
composites [151–153]. Their results proved that the thermal acetylation reacts with hydrophilic hydroxyl groups of the
and mechanical properties of PF were directly affected by alkali fibre to generate esterification that reduces its hydrophilic
treatment that provides a rough surface topography through nature by absorbing moisture from the fibre [161]. As a
removal of natural and artificial impurities. Barreto et al. [154] result, after acetylation, the dimensional stability was im-
found that composites containing alkali-treated fibre bundles proved as well as the dispersion of fibre into polymeric
have better mechanical properties than those with untreated matrices, thus increasing the hydrophobic nature of the fibre
fibre bundles. However, the effectiveness of the alkali treat- due to the substitution of hydroxyl groups with acetyl
ment of PF can only be increased further by conducting the groups. Generally, alkaline treatment was done before
treatment at elevated temperatures as the heat energy would acetylation and has been found to reduce the impact strength
provide some additional catalytic effect in breaking the hy- and stiffness and improve the interfacial bonding, as well as
drogen bonds within the fibrils [155]. In this treatment, the the dimensional stability and thermal stability and resistance
concentration of the alkali solution, operational temperature, to fungal attack in PF composites. The mechanical and other
temperature treatment time, material strength, and the applied physical properties of the composite are usually dependent
additives are parameters to be considered. on the fibre content, which also determines the possible
amount of coupling agents in the composite [162].
9.2. Coupling Agent. Coupling agents have been developed
to improve the interfacial bonding for adhesion that relates 9.2.3. Graft Copolymerization. Graft copolymerization is a
to enhancement of the mechanical strength and durability prominent cross-linking agent used since 1943 to chemically
of the fibre-matrix. The coupling agents are compatible increase the compatibility of PF or wood with a suitable
with the fibre-matrix in reducing water absorption, elim- solution that forms free radicals on the cellulose molecules
inating the leaching effect and enhancing the wettability of by reaction through selected ions with hydrophobic matrices
fibres by the polymer chains [156]. The surface modifica- [163]. The method involves the grafting of various vinyl
tion procedures applied to PF using coupling agents such monomers, acrylonitrile, and methyl methacrylate [164].
as silanes, acetylation, and graft copolymerization are Through this method, functional groups that can interact
intended to enhance the chemical bonding of the oxide with cellulose or other constituents of the natural fibres are
groups on the fibre surface with the polymer molecules that grafted into the same polymers or polymers that have a
link together the hydrophilic fibres and hydrophobic resemblance to a given matrix [56]. Thus, the grafted systems
polymers for excellent composite mechanical properties are able to act as bridges to minimize the mismatches in
[157]. This confirms previous findings by Bledzki and polarity between the hydrophilic fibres and hydrophobic
Gassan [8] that the chemical treatment due to coupling matrices. The functional groups that are actively used
10 Advances in Civil Engineering
Table 4: Advantages and disadvantages of fibre chemical treatments.
Treatment Advantage Disadvantage Remarks Ref.
More than 6% increment of The types and concentration of
Rough fibre surfaces provide better
alkaline concentration and longer the alkaline solution, time of
mechanical interlocking and
Alkaline than 24-hour soaking periods treatment, and the temperature [128, 129]
stronger interfacial strength
damage the fibres and reduce used for modification affected the
between the fibre-matrix
tensile strength efficiency
Increases toughness and reduces Treated fibre tensile strength
Silane treatments could be more
water adsorption of the fibres decreased due to cellulose
Silanes effective when combined with [130, 131]
resulting in hydrophobic microfibrils receiving less support
physical treatment
composites against tensile loading
Increasing the degree of Acylation may not be as effective
acetylation decreases the as alkaline treatments for
Increases crystallinity and reduces
Acylation mechanical properties due to improving the interfacial [132, 133]
water adsorption
degradation of cellulose and interaction between fibres and
cracking of fibres matrices
Increases fibre-matrix adhesion,
Maleic anhydride (MA) is the
Graft reduces water adsorption, thermal High initiator concentration,
most prominent functional group
copolymeriza- stability, and enhances the temperature, and fibre loading [134, 135]
due to cost, performance, and
tion mechanical properties of the influence the grafting effect
commercial availability
composites
Bleaching effects have a positive
Reductive Fibres are highly hydrophobic and Not as efficient as oxidative reaction with a 1–2% dosage of
[136, 137]
bleaching bright bleaching reductive bleaching agents and
higher than 60°C temperature
Expensive treatment requires
No separate pretreatment is needed high temperatures, and fibre- Combination treatment with
Oxidative due to the combined effect of coloured compounds are coupling agents is needed to
[138, 139]
bleaching bleaching and cleaning with fibre destroyed and cannot reform due modify the wet-out and interfacial
quality intact to the permanent bleaching bonding
process
Differences in enzyme quality
Effectively used to produce
including contamination of Variation types of fibre and
homogenous fibre surfaces with
enzymes during preparations enzymes have different reactivity
Enzyme improved thermal properties by [140, 141]
with others of varying specificity to the targeted components
removal of the hygroscopic pectin
will require careful and detailed because of geometrical shapes
and hemicellulose content
analysis
Possibility of forming mechanical
It may facilitate both mechanical
and chemical bonding at the fibre
Increases crystallinity, thermal interlocking and the bonding
surface is mainly dependent on
Peroxide stability, and mechanical reaction due to the exposure of [142, 143]
the surface morphology and
performance the hydroxyl groups to the
chemical composition of the
chemical
fibres
Rough fibre surfaces provide better
4% is the maximum dosage of the
mechanical interlocking and The use of hazardous chemicals
Permanganate compound to yield tensile [144, 145]
stronger interfacial strength causes environmental pollution
strength
between the fibre-matrix
include methyl groups, isocyanates, triazine, benzoylation, dissolving and eliminating lignin and hemicellulose from
maleic anhydride, and organosilanes. However, the best the matrix surrounding the cellulose microfibrils [165].
functional group for compatibility by graft copolymerization Bleaching treatments are environmentally safe, and most of
is maleic anhydride (MA) due to cost, performance, and the PF used for textile fabrics are bleached with hydrogen
commercial availability [64]. peroxide. The temperature and pH must be controlled
when using hydrogen peroxide because high temperature
and alkalinity will damage the fibres. The effect of bleaching
9.3. Bleaching. Bleaching treatments including peroxide on PF may be seen visually as a change in appearance and
bleaching, alkali and enzyme treatments, and biobleaching other aesthetic properties, but it may result in deliberate
can improve the appearance of bast fibres. The purpose of loss of fibre tensile strength [166] due to the loss of lignin as
the bleaching was to isolate individual microfibres through a cementing material and result in a decrease in the tensileAdvances in Civil Engineering 11
Table 5: Alkali effect on tensile properties of PF.
Treatment
Fibres Tensile strength (MPa) References
Chemical Physical
— 289
Untreated 5% 355
Kenaf NaOH soaked for 24 h Oven dried at 220°C (10 h) [97]
Treated 10% 261
15% 213
— 16.8
Untreated 3% 17.5
Kenaf NaOH soaked for 24 h Oven dried at 80°C (10 h) [148]
Treated 6% 17.6
9% 16.8
— 755
Untreated 5% 847
Abaca NaOH soaked for 0.5 h Oven dried at 80°C (2 h) [146]
Treated 10% 840
15% 840
— 463.84
Untreated 5% 605.61
Mulberry NaOH soaked for 0.5 h Dried at room temperature (48 h) [149]
Treated 10% 532.24
15% 519.30
— 350
Untreated 2% 370
Date palm NaOH soaked for 1 h Oven dried at 80°C (24 h) [150]
Treated 5% 460
10% 250
strength of the composite. Generally, bleaching was divided matrix and improved the mechanical properties of the
into reductive and oxidative bleaching. composites. Yi et al. [168] found that the crystallinity and
thermal properties improved after separating hemp fibre
bundles using enzymes into individual bundles. It also in-
9.3.1. Reductive Bleaching. Reductive bleaching is a tem-
fluences the structure, chemical composition, final fibre
porary process performed on protein fibres containing high
quality, and properties of the fibres [169]. Enzyme treatment
cellulose using sodium dithionite (Na2S2O4) associated with
is progressively popular, with benefits related to environ-
the reduction of chromophores (coloured fibres) to leuco-
mental friendliness [170], and the reaction catalysed from this
chromophores (uncoloured fibres). Although it is stable
treatment is very specific, focusing on the particular per-
under most conditions, it will decompose in hot water
formance required. It can be recycled after each use [62], but
and acid solutions. Due to significant losses in durability,
the discharge will affect the environment [171].
reduction bleaching is rarely explored for surface modifi-
cation of PF for composite purposes [64]. This enables
oxidative bleaching to be frequently used treatment method
9.5. Peroxide. Peroxide treatment is a process similar to the
using sodium hypochlorite (NaClO), hydrogen peroxide
initiation step of the free-radical polymerization, which
(H2O2), or sodium chlorite (NaClO2). NaClO attacked the
induces adhesion in cellulose fibre-reinforced thermoplastic
hydroxyl groups in the lignin to form aldehyde groups
composites. Chemicals that can be used as initiators of the
(CHO) that can reduce the lignin content, whereas existing
polymerization include benzoyl peroxide (BPO) and
aldehyde groups will cause the degradation of cellulose in
dicumyl peroxide (DCP) [172]. An alkali pretreatment is
NF. However, NaClO2 bleaching can reduce lignin and
required before the peroxide treatment to separate wax,
pectin with minimal amounts of cellulose degradation at a
hemicellulose, and lignin. Since the peroxide treatment is the
reasonable cost compared to NaClO and H2O2. However,
first step of free-radical polymerization, it works best as an
the bleaching process using H2O2 is gradually replacing
interface modifier when the curing mechanism of the matrix
other oxidative bleaching processes due to its environmental
polymer is free-radical polymerization, whilst permanganate
friendliness, although the chemical cost is higher than many
is not technically a peroxide treatment, but it has a similar
other bleaching agents with similar capabilities [167].
treatment method. The permanganate treatment uses the
oxidation property of KMnO4 to reduce the hydrophilic
9.4. Enzymes. Enzyme or biological modification is an ef- nature of cellulosic fibres. The degree of reduction in hy-
fective method for fibre surface treatment through removal of drophobicity is found to grow with increasing KMnO4
lignin and hemicellulose, and it requires lower energy input. concentration. Peroxide treatments can significantly reduce
The fungus or bacteria release enzymes that can deteriorate or the water absorption of natural fibres/fillers, which in turn
remove the pectinase glue that bonds the fibre bundles to can improve the interfacial adhesion between fibres/fillers
release the cellulosic fibres. The treatment resulted in an and hydrophobic polymer matrices [173]. Meanwhile, the
increase in the surface hydrophilic interface between the fibre- tensile properties of peroxide-treated natural fibre-12 Advances in Civil Engineering
reinforced thermoplastic composites showed clear im- interfacial adhesion between the fibre-matrix and enhance
provement. On the other hand, the rate of peroxide de- good mechanical properties of composites.
composition can also negatively affect the mechanical Physicochemical surface treatments provide good
properties of treated composites. compatibility and interface bonding in the modification of
PF. The effort to find the most suitable physicochemical
10. Physicochemical Treatments combination for PF surface modification must be contin-
uous as the reinforcement fibre is the major contributor to
The combination of chemical and physical treatments is the mechanical properties of the composite. When the effects
known as a physicochemical treatment and combines of various fibre treatments were put into consideration for
physical treatments with chemical treatments to produce selecting suitable fibres, it can be seen that physical or
support to the chemical reactions and improved separation chemical surface modification treatment alone can be ap-
of fibre bundles [174]. These types of treatments provide plied depending on what properties we want to achieve.
clean and fine natural fibres or fibrils with high cellulose However, it is suggested to use physicochemical treatment
content. The mechanical properties of these fine fibres are for potential excessive changes in fibre surface properties
close to those of pure cellulose fibres, which can significantly with improvement in the compatibility of different matrices
improve the appearance and mechanical properties of PF and also to overcome the hydrophilic nature of PF surface
[64]. properties. Moreover, the surface free energy, the me-
chanical interlocking at the interface of the composite, and
11. Summary the mechanical properties of the composites also increase
due to improvements in fibre-matrix adhesion related to
The combination of alkali and thermal treatments for surface various types of physical and chemical modifications.
modification of PF is popular among the various combi-
nations of chemical and physical techniques that provide
better adhesion between the fibre-matrix. Chemical treat-
Conflicts of Interest
ment plays a major role in the interfacial properties of the The authors declare that they have no conflicts of interest.
fibres, and thermal treatment improves the surface area
exposed for treatment and matrix interaction by assisting in
the separation of the fibre bundles. Thermal treatments also Acknowledgments
increase the hydrophobicity of lignocellulose fibres, crys-
The authors acknowledge the Ministry of Higher Education
tallinity, and dimensional stability. While NaOH is the most
Malaysia for financial support under Fundamental Research
common alkaline solution used in the physicochemical
Grant Scheme (FRGS/1/2016/TK06/UKM/02/2) and Uni-
treatment, the use of sodium bicarbonate (NaHCO3) started
versiti Kebangsaan Malaysia (UKM) under grant AP-2015-
getting attention lately due to its similar effect on PF and cost
011 and also the facilities provided by the Civil Engineering
effectiveness. High temperatures have been reported ranging
Programme and Smart and Sustainable Township Research
from 100°C to 200°C. However, low temperatures below
Centre, UKM.
100°C had a significant effect on the adhesive properties of
PF and showed that this parameter affects the fibres as
reinforcements. In summary, the combination of both References
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